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| United States Patent |
5,602,516
|
|
Parfitt
|
February 11, 1997
|
Asymmetrical notch filter
Abstract
A dual stage filter providing a relatively deep notch whose attenuation
versus frequency curve is asymmetrical. The filter consists of a helical
transmission line one end of which is connected to electrical ground. The
opposite end is connected to a variable piston capacitor whose remaining
end is also connected to ground. The ungrounded junction of these two
components is connected to an input connector through a series capacitor.
An identical circuit is arranged and connected to the output connector.
These two circuits are isolated from each other by a conductive partition
and housing. A series inductor connects the input and output connector. A
matching capacitor is connected in parallel across a portion of the series
inductor.
| Inventors:
|
Parfitt; Dale R. (6869 Bayshore Dr., Lantana, FL 33462)
|
| Appl. No.:
|
543415 |
| Filed:
|
October 16, 1995 |
| Current U.S. Class: |
333/176; 333/202 |
| Intern'l Class: |
H03H 007/01 |
| Field of Search: |
333/175,176,202
|
References Cited
U.S. Patent Documents
| 4186359 | Jan., 1980 | Kaegebein | 333/134.
|
| 4449108 | May., 1984 | Endo et al. | 333/202.
|
| 4586007 | Apr., 1986 | Ciszek | 333/175.
|
| 4901043 | Feb., 1990 | Palinkas | 333/176.
|
| 5032807 | Jul., 1991 | Petrovic | 333/176.
|
| 5202656 | Apr., 1993 | Clark et al. | 333/176.
|
Primary Examiner: Lee; Benny
Assistant Examiner: Bettendorf; Justin P.
Claims
What is claimed is:
1. An asymmetrical notch filter comprising: a helical transmission line
whose bottom end is connected to a conductive housing that is at
electrical ground; and top end of said helical transmission line connected
to an input terminal through a fixed capacitive means; a variable
capacitor is connected in parallel with said helical transmission line; a
second helical transmission line whose bottom end is connected to said
conductive housing and top end connected to an output terminal through
second capacitive means; a second variable capacitor is connected in
parallel with said second helical transmission line; first and second
helical transmission lines electrically isolated from each other by a
conductive partition and said conductive housing; said input and output
terminals connected by an inductor.
2. The filter of claim 1 wherein a capacitive means is connected in
parallel with a portion of said inductor connecting said input and output
terminals.
Description
BACKGROUND--FIELD OF THE INVENTION
Background of Invention
This invention relates to electronic notch filters and more particularly to
VHF notch filters with a relatively sharp transition between the notch
frequency and the pass frequencies and an asymmetrical frequency vs.
attenuation curve.
BACKGROUND--DESCRIPTION OF PRIOR ART
It is well known that the front end amplifying stages of receivers are
limited in the strength of signal(s) they can linearly amplify before
distortion begins to occur. This distortion is typically called
intermodulation or cross modulation distortion. The most common method to
diminish intermodulation is to limit the band of frequencies that are
"seen" by the receiving amplifier. Bandpass filter topology has
historically been the filter of choice. Either the filter has a bandwidth
sufficient to pass the frequencies of interest or more expensive and
complex tracking bandpass filters may be employed. In this filter, the
passband is considerably narrower than the spectrum to be received, but
the filter is electronically or manually tuned along with the receiver so
that the bandpass filter's center frequency is the same as the frequency
of the receiver.
The 2 meter amateur band (144-148 megaHertz) suffers from a unique form of
intermodulation. Pager transmitters are assigned to multiple channels in
the 152-153 megaHertz region and 157-159 megaHertz region. They typically
run high power (300-350 watts) and transmit at a high duty cycle (time on
the air/time off the air). This makes them a primary candidate to cause
intermodulation in susceptable receivers. If, in the general third order
intermodulation formula (Fim=F1.+-.F2.+-.F3) F1 and F2 are chosen as
pagers in the 152-153 region and F3 is chosen to be a pager channel in the
157-159 region, the sum of F1 and F2 minus F3 places the intermodulation
products throughout the 2 meter amateur band. The problem has gotten worse
in recent years as the radio manufacturers have included general coverage
VHF receive (120-174 mHz) in the transceivers. Wide-band bandpass filters
have replaced the previous filters centered around 144-148 megaHertz. A
few radios employ tracking front end bandpass filters but because of the
necessity of small size, these filters have limited rejection capabilities
of signals so close to the 2 meter band.
Solutions to this problem to date have been to add on an external bandpass
filter. These vary from large quarter wave cavities (typically 19"
tall.times.5" diameter)) that need to be tuned if the receive/transmit
frequency is changed appreciably to large multiple cavity reentrant
bandpass filters (Sinclabs Ontario Canada). These filters typically cost
several hundred dollars or more.
Another solution is a smaller bandpass filter distributed by Tucker
Electronics of Dallas, Tex. This filter, in order to achieve sufficient
selectivity has appreciable insertion loss necessitating that it be
switched out during transmission. This is accomplished by applying +12
volts to a bypass relay during transmit.
In addition to the above mentioned problems, all of these approaches
severly limit the range of frequencies that can be received when the
filter is connected. The receiver equipped for general coverage VHF
receive is now limited to 144-148 megaHertz or less. Another problem is
encountered when these filters are used on dual band radios (usually 2
meter and 70 centimeter) having a single antenna connection. Because the
bandpass filter by definition is very lossy at frequencies removed from
its bandpass, it must be taken off the radio before the 70 centimeter
portion of the radio can be used. This can be inconvenient and time
consuming.
As can be seen from the forgoing, solutions to date have been of the
bandpass filter type. These solutions fail to recognize that the paging
transmitters are nearly the sole cause of intermod not only to the 2 meter
amateur band but to VHF scanner receivers and 2 way radios operating in
the 150-170 megaHertz spectrum. If a filter eliminates one or more of the
terms in the intermodulation formula (F1,F2), even though F3 remains, the
intermodulation will disappear.
Although notch filters are known (Ciszek, U.S. Pat. No. 4,586,007 and
Petrovic U.S. Pat. No. 5,032,807) and asymmetrical notch filters (Endo et.
al. U.S. Pat. No. 4,449,108) none contain all of the aforementioned
features along with relative simplicity of construction as will be seen.
For instance, although the circuitry of the Ciszek patent may at first
blush seem similar, the actual operation of the circuit as will be seen,
is entirely different. In the Ciszek filter the parallel tanks (36, 42)
are resonated at the notch frequency and attenuation takes place as a
result of phase cancellation provided by capacitor 44.
Asymmetrical notch filters generally contain a lumped or distributed
reactive element in series with the input-output connection (Endo et al
U.S. Pat. No. 4,449,108 Kaegebein U.S. Pat. No. 4,186,359). It is this
reactance that yields the asymmetry. However, at frequencies above the
notch frequency (in the instance of a series inductive element) the series
reactance becomes appreciable and through loss increases with increasing
frequency (see FIG. 5). Just the opposite is true with a capacitive series
element (see FIG. 6). The present invention provides a simple solution to
this problem for a set of frequencies well above the notch frequency. This
response is desirable when the filter is used with dual band radios as
will be described.
OBJECTS AND ADVANTAGES
Accordingly the filter described in this application has the following
advantages over the prior art.
The filter is small compared to cavity types. The preferred embodiment
measures 1".times.2".times.2.5". The asymmetrical nature of the notch
allows a very sharp transition from the notch frequency to the pass
frequency with relatively uncomplicated circuitry. Typically, a notch in
excess of 45 decibels is achieved at 152.5 megaHertz (the center of the
lower of the two paging spectra previously mentioned) while loss of less
than 0.2 decibels is achieved throughout the 2 meter amateur band. This
compares with losses of 1-6 decibels in the bandpass versions. As shown in
FIG. 1 the notch rises much more slowly on the high frequency side. This
is desirable in order to notch as many of the pager channels as possible.
V.S.W.R. (voltage standing wave ratio) is typically less than 1.2:1
throughout the 2 meter band. Because of these favorable characteristics,
there is no need to switch the filter out during transmission as is the
case with some competing models. Frequencies above or below the notched
spectrum are still received. This allows users of radios equipped with
general coverage VHF receive to still receive all frequencies except those
in the notch band. Although the loss begins to increase for frequencies
well above the notch frequency, in the preferred embodiment the addition
of a single capacitor (22) makes the filter transparent at 70 centimeters
allowing it to be used with the aforementioned dual band radios. Finally,
cost is considerably less than the previously mentioned devices.
Still further objects and advantages will become apparent from a
consideration of the ensuing description and accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of a filter according to my invention.
FIG. 2 is a perspective view of the invention with portions of the outer
housing removed for clarity
FIG. 3A is a schematic representation of the preferred embodiment
FIG. 3B is a schematic representation of another embodiment of the filter.
FIG. 3C is a schematic representation of a 3rd embodiment of the filter
FIG. 4 is a graph of attenuation vs. frequency of the filter of FIG. 3A
FIG. 5 is a graph of attenuation vs. frequency of the filter of FIG. 3B
FIG. 6 is a graph of attenuation vs. frequency of the filter of FIG. 3C
REFERENCE NUMERALS
10 helical resonator
10A helical resonator
12 housing
14 piston capacitor
14A piston capacitor
16 RF connector
16A RF connector
18 capacitor
18A capacitor
20 series inductor
22 matching capacitor
24 conductive partition
26 series capacitor
Preferred Embodiment--Description
FIG. 1, FIG. 2--Asymmetrical Notch Filter.
FIG. 1 is a perspective view of the present invention showing the RF
connectors 16, 16A and the tuning adjustment portions of piston capacitors
14, 14A. FIG. 2 is a perspective view of the present invention with a
portion of the housing 12 removed for clarity. The same device is shown
schematically in FIG. 3A. Helical transmission line 10 has its first end
connected to housing 12 which is at electrical ground. A second end of
helical transmission line 10 is connected to first end of piston capacitor
14. Second end of piston capacitor 14 is electrically connected to housing
12. First end of piston capacitor 14 is joined to RF connector 16 through
capacitor 18. A second circuit comprised of helical transmission line 10A,
piston capacitor 14A, capacitor 18A and RF connector 16A are configured
identically to their unsubscripted counterparts as shown schematically in
FIG. 3A. A series inductor 20 joins RF connector 16 to a second RF
connector 16A. Matching capacitor 22 is shunted across a portion of series
inductor 20. Conductive partition 24 electrically isolates the two helical
circuits. Typically, housing 12 is comprised of two 1".times.1" sections
of square brass tubing 2.5" long soldered together along common walls to
form partition 12. Partition 12 has an elongated notch milled into its
upper end to permit series inductor 20 to pass unobstructed from RF
connector 16 to RF connector 16A. Inductor 20 is typically made up of
seven turns of 18 gauge wire 0.125" in diameter and 1" long. Matching
capacitor 22 is connected in parallel with the first three turns of series
inductor 20.
Helical transmission line 10 and helical transmission line 10A are
constructed according to the well known equations for helical transmission
lines as set forth in The Handbook of Filter Synthesis by A. I. Zverev for
instance. In particular, the helical transmission lines of the present
invention are constructed of 7 turns of #16 gauge copper wire 0.5" in
diameter and 1.0" in length. Piston capacitor 14 and piston capacitor 14A
are multiturn coaxial type variable capacitors with a typical tuning range
of 0.5-5 pF. They are so mounted as to allow adjustment from the exterior
of housing 12. This is clearly shown in FIG. 1. A suitable type used in
the present invention is manufactured by Stetnor Trush. Matching capacitor
22 is a ceramic disc capacitor having a value of 10 pF. Capacitors 18 and
18A are 3.0 pF NPO temperature stable disc capacitors.
Preferred Embodiment--Operation
Most commonly, helical transmission lines are used in their parallel
resonant mode where either they are self resonant or are tuned to parallel
resonance by the addition of a small variable capacitance in parallel with
the helical transmission line. In the present invention helical
transmission line 10 is less than a quarter wave long and therefore looks
like a very high Q inductor. Piston capacitor 14 allows for small
adjustments to the length of helical transmission line 10. The net result
of this combination appears as an adjustable, high Q inductor one end of
which is at electrical ground while the opposite end is connected in
series with capacitor 18. Piston capacitor 14 is then adjusted to yield a
high Q series resonant circuit at the desired notch frequency. In order to
preserve the high Q nature of helical transmission line 10, the value of
piston capacitor 14 is relatively small, typically less than 1 pF.
Components 10A, 14A and 18A function in a manner identical to their
unsubscripted counterparts. Taken separately, either of these two series
resonant circuits yields a relatively wide symmetrical notch of
approximately 20 dB. When connected together by series inductor 20 as
shown in FIG. 3A the resultant notch is in excess of 45 dB and
asymmetrical as shown in FIG. 5. When the two circuits are adjusted for a
notch at 152.5 mHz. (approximately the center of the lower paging
spectrum) the filter loss is less than 0.2 dB below 150 mHz. V.S.W.R. in
the 2 meter amateur band is typically less than 1.2:1.
Inductor 20 causes the filter to be asymmetrical as shown in FIG. 5. This
asymmetry is important in that it yields surprisingly low insertion loss
and V.S.W.R. at frequencies below and close in to the notch frequency. At
frequencies well above the notch frequency the reactance of series
inductor 20 causes substantial V.S.W.R. and insertion loss. Because many
of the amateur transceivers are dual band (2M and 70 cM) it would be
desirable to have the filter be transparent at 70 cM (420-450 mHz.). The
function of matching capacitor 22 is to cause the network comprised of
series inductor 20 and matching capacitor 22 to be series resonant at 70
cM forming a very low impedance connection from input connector 16 to
output connector 16A. At VHF frequencies, the reactance of matching
capacitor 22 is sufficiently high as to be invisible. Thus at VHF
frequencies the circuit appears as in FIG. 3B. The two series resonant
circuits formed by 10, 14, 18 and 10A, 14A, 18A are essentially high
impedances at UHF and are invisible to the 70 cM energy. The result is an
insertion loss of less than 0.2 dB and a V.S.W.R. of less than 1.5:1
across the 70 cM amateur band. This response is clearly shown in FIG. 4.
Other Embodiments
Notch Filter with the asymmetry curve reversed--Description
If dual band operation is not required then the filter of FIG. 3B may be
employed. Matching capacitor 22 is omitted. The resultant frequency versus
attenuation curve is unchanged at VHF frequencies but now the filter has
some loss at all frequencies above the notch frequency and would not be
suitable for transmitting at these frequencies. Still, this embodiment is
useful for VHF only radios that are frequently encountered in both the
amateur and commercial radio markets. The altered attenuation versus
frequency curve for this embodiment is shown in FIG. 3B
Notch Filter For Use With Scanner Radios--Description
FIG. 3C shows yet another embodiment of the present invention. Scanner
radio enthusiasts are generally not interested in listening to the 2 Meter
amateur band or do so with an amateur transceiver. However, the
frequencies just above the pager assigned frequencies (154-174 mHz.) are
heavily populated by public service radios that are of great interest to
scanner enthusiasts. The shape of the filter of FIG. 3A, although usable,
would not be the best solution for this application because of its
appreciable loss at frequencies just above the notch frequencies. The
filter embodiment of FIG. 3C yields a filter whose basic response is the
mirror image of that of FIG. 3B. This is accomplished by removing series
inductor 20 and substituting series capacitor 26. Typically this is a
ceramic disc capacitor whose value is 18 pF. The frequency versus
attenuation curve of this embodiment is shown in FIG. 6.
Conclusions, Ramifications, and Scope
Accordingly, it can be seen that I have provided a novel filter that
effectively eliminates intermodulation distortion in 2 meter amateur
transceivers, VHF radios, and scanner receivers. The present invention has
advantages over current solutions in that it is generally smaller, is
usable on dual band radios, has lower insertion loss, does not require the
filter to be switched out during transmit and is more economical.
Although the description above contains many specificities, these should
not be construed as limiting the scope of the invention but as merely
providing illustrations of some of the presently preferred embodiments of
this invention. Various other embodiments and ramifications are possible
within it's scope. For example, although the filter is specified to
eliminate the paging transmitters in the 152-153.5 mHz range it is
adaptable to other frequencies experiencing similar problems.
Thus the scope of the invention should be determined by the appended claims
and their legal equivalents, rather than by the examples given.
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